The present invention provides an improvement to the basic disc drive spindle motor in which ball bearings are utilized to journal a rotor about a shaft in a spindle motor. The embodiments of the present invention are designed to implement a magnetic bearing in a disc drive providing for a unique application of the known technology to improve disc drive motor performance.
Conventional mechanical bearings used in conjunction with high rotational speed devices are subject to metal wear, vibration/shock and friction problems. Metal wear occurs when the metal to metal contact surfaces of the conventional ball bearings rub against the interior or exterior races (or against the rotating body in a raceless bearing) of the ball bearing assemblies. Conventional bearing systems provide lubricants to minimize the wear due to surface abrasions. However, at start up, when the lubricants have tended to settle, discernable metal to metal contacts will arise, and over life wear will result for bi-directional or uni-directional motion. While lubricants help to alleviate some of the wear problems, their introduction into the contamination sensitive disc drive environment poses a second problem, namely out-gassing and particulate contamination. In a disc drive, particulate contamination must be minimized due to potential damage and distortion of read write transducers which fly at small gaps on the order of 2.5 micro inches above the disc surface. As data densities increase lower "fly" heights will result thereby magnifying the problem. Additionally, as particulate contamination increases, data may be incorrectly written or irretrievable, thereby dramatically affecting system integrity and performance. Thus in conventional mechanical bearing systems, conventional means for minimizing metal wear give rise to other delirious effects.
Vibration and shock problems arise in two particular contexts. First, disc drives are designed to be resistant to levels of shock to allow for the handling of the disc drive itself as well as the particular use of the disc drive in the given working environment. Particularly the removable and portable disc drive systems must be designed so as to accommodate for everyday knocks and shocks without disc system failure. The conventional bearing systems used in disc drives must be able to sustain these everyday shocks while maintaining system integrity. Because of the surface-to-surface contact and surface stress between the balls and races found in a conventional bearing system, the aforementioned everyday shocks will often result in abrasions or deformities to the ball bearing surfaces, the effects of which will be discussed below in conjunction with the friction problem.
Secondly, recognizing the shock requirements that the disc system must perform under, another shock related problem arises, namely shock testing. During the qualification phase of production, each disc drive is subjected to a shock test in order to assure that each disc drive will be able to withstand the rigors of the operating and non-operating environment described above. Shock loads result in high surface stress between the contacting surfaces which can exceed the elastic deformation of the individual materials causing additional wear, ultimately leading to bearing failure.
The third problem found in the use of mechanical bearing systems is interrelated and directly attributable to metal wear and surface quality because of the high precision of machined bearing components, namely friction. Friction is a measure of the resistance to motion of the ball bearing, and must be overcome by the motor in order to begin or maintain the ball bearing in motion. Therefore, extra motor power must be allotted to compensate for the effects of friction and also budgeted for increases in friction over the life of the mechanical bearing system. Friction in a conventional ball bearing has many components and is attributable in part to the ideal (initial) roundness of the ball bearing used, the viscosity of the lubricant selected, and the amount and severity of surface irregularities developed over life. As metal wear and other surface irregularities arise, the normally smooth surface of the ball bearing and raceways becomes pitted and scratched resulting in an increased frictional force which opposes motion of the bearing dramatically at start up (a motor torque problem) and also during normal operation. Any frictional forces will result in an inherently inefficient use of motor power, and may ultimately result in the failure of the bearing system, the spindle motor or other disc drive systems.
Another problem arises from the use of conventional bearing systems, acoustic noise. Conventional bearing systems are inherently noisy, and as surface irregularities develop in conventional ball bearings the acoustic noise generated by the spinning bearings is both clearly audible and potentially bothersome to the average consumer. The computer disc drive industry has moved away from the whirling noisy early disc drive offerings, and now is very conscious of any acoustic noise sources. As such, any improvement over an inherently noisy conventional ball bearing system is desirable for hard disc drive applications.
Another problem found with the use of conventional bearing systems is the vibrations the rolling bearing systems induce in the disc drive assembly. As the ball bearings rotate, vibrations and inaudible noise will be induced at varying frequencies due to the dynamics of the movement and geometric shape of the bearing components of the ball bearing system. As surface damage induced irregularities arise, bearing defect frequencies may appear, and the level of noise and/or vibration may increase. Disc drive designers are aware of the deleterious effects of such noise sources and design around such frequencies, often employing filters or other dampening means which are both costly and space prohibitive. Bearing systems which would not "age", which could be characterized specifically at design inception would allow for better and more efficient disc drive design solutions
Finally, in a disc drive, the mechanical bearings must perform consistently over their rated life, any failure resulting in failure of the entire system. Performance of the mechanical bearings in a disc drive may be measured by various parameters including wear and losses (due to friction) as described above, as well as other disc positioning performance parameters such as radial and axial stiffness, susceptibility to repeated shock and to non-repeatable run-out.
Disc positioning parameters refer to the ability of the disc drive spindle motor to rotate a given disc consistently in a given plane while maintaining the spatial relationship of the disc with respect to the remaining disc system components. Stiffness in either the radial or axial direction is a measure of the bearing system's (or pivot's) ability to maintain the relationship of the spinning disc with the remaining disc drive components. Axial stiffness in a axially aligned spindle motor provides for resistance to compression or tilt of the fixed disc about the bearing system due to external axial shocks. The higher the axial stiffness, the less likely tilt will occur, and accordingly, less likely that the read/write heads will come into contact with the spinning disc upon the introduction of an external axial shock. Radial stiffness in an axially aligned spindle motor provides for resistance to radial movement of the spinning disc toward or away from the shaft as radial forces are exerted on the disc drive. The higher the radial stiffness, the less likely that disc positioning errors will occur. This is particularly important in the higher density disk drives of today, where with the increased track densities on the order of 5 micro-inches, even the smallest deviation of disc radial location will result in data errors. Because of the contact-contact configuration of conventional bearing systems, directional stiffness is easily achievable in systems employing conventional bearings. As such the use of mechanical bearings and pivots provides a high degree directional stiffness performance.
Non-repeatable run-out particularly is a performance measure of the disc drive spindle motor's ability to place the magnetic heads over an identical spot on the spinning disc consistently over time. Deviations in the location of the read/write heads with respect to the spinning disc will yield data losses and/or irretrievable data. As such a spinning disc, and particularly a bearing system for journaling a disc about a rotating shaft, must consistently perform over life, repeatedly positioning the read/write heads with respect to the spinning media. Conventional bearing systems degrade over life, necessarily resulting in poorer non-repeatable run-out performance of the bearing systems.
Magnetic bearings are well known. Particularly, the use of magnetic bearings in motors has heretobefore been known. Magnetic bearings provide a low friction means of journaling components, however they also provide only limited directional stiffness and stability. Significantly, the stiffness provided by a conventional bearing system is at least two orders of magnitude greater than a magnetic bearing of the same overall size. In order to achieve comparable directional stiffness performance, a magnetic bearing system would occupy a significantly larger area, or require active field-intensifying components, the sum of which would deleteriously affect overall disc drive system size, cost and performance. Accordingly, magnetic bearings were heretobefore unknown in the disc drive industry where cost, component size and bearing system stiffness were of paramount concern. Notably, a passive magnetic suspension system, one in which no active feedback measures are employed, is now particularly attractive because of improved magnet energy to cost ratios, the subject matter of which is exploited in the present application.